Different controls (modifications) are added to the ellipsoid in order to construct the datum, which is specialized for a specific geographic regions (such as the North American Datum).

• 球面模型适用于小比例尺地图，例如世界地图集或地球仪等，因为在这种比例尺下球面与椭球面间的误差基本可以忽略不计。